The present invention relates to a novel micro-structure developed in a cast austenitic stainless steel alloy and a braze cycle heat treatment thereof. Specifically, the present invention is directed to an alloy based on a multicomponent Fe-Cr-Mn-Mo-Si-Nb-C system which consists of an austenitic iron solid solution (.gamma.) matrix reinforced by finely dispersed carbide phases and a braze cycle heat treatment to produce this micro-structure.
Austenitic stainless steels are most commonly composed of 12-28 wt% chromium and 4-22 wt% nickel additions which impart corrosion resistance far superior to common steels as well as stabilize the face-centered cubic (FCC) lattice. These stainless steels are commonly strengthened by solid solution mechanisms as well as by precipitation of Cr.sub.23 C.sub.6 carbides as a fine dispersion. These carbides which are initially coherent with the FCC matrix coarsen rapidly upon exposure to elevated temperatures in excess of 650.degree. C. This exposure to elevated temperatures weakens the micro-structure and limits their use at temperatures well below 650.degree..
A need exists for a low-cost cast iron-based alloy that meets the requirements of high strength and thermal fatigue resistance to approximately 800.degree. C., compatibility and low permeability with hydrogen, elevated temperature oxidation-corrosion resistance, and contains a minimum amount of strategic elements.
The present invention thus addresses the development of a cast iron-based alloy in combination with other low-cost low strategic value elements such as manganese, silicon and carbon and avoids the additions of cobalt or nickel. Because of the high carbon levels (greater than 1.0 wt%), chromium additions can be made using high-carbon ferro-chromium derived from chromite deposits in the Western United States. Until recently, these chromite deposits had been considered too low grade for use in the manufacture of stainless steel because of their lower chromium-to-iron ratios of 1.5:1 and their higher carbon content.